上述单词均可用作违约之诉的辩护理由。它们的区别在于 impossibility (也称为 impossibility of perfonnance)为合同的“履行不能”，是合同法中一原则，指因无法抗拒的情况发生，如合同必要的执行人死亡、合同变为非法等[1]。impracticability为 “履约的不实际性”（也称为 commercial impracticability; impracticability of performance),它所指的范畴比 impossibility of performance广，包括因某种无法预见的意外事故的发生，导致履约极端困难、危险或费用太高，从而可以免除履约责任的情况[2]; 如在销售合同中，因无法预见的情况导致所合同规定的货物不存在等均属于impracticability的范畴。Frustration为“合同受挫原则”（也称为 frustration of contract ； frustration of purpose；frustration of the venture)，其是普通法一原则，指合同双方当事人均认为，或合同条款规定某物的存在为履行允诺的必要条件，在履约时如果该物因意外事故或不属于双方当事人的过失而不复存在，允诺方可免除履行允诺的义务[3]。

[1] “A fact or circumstance that excuses performance because (1 ) the subject or means of performance has deteriorated, has been destroyed, or is no longer available；(2) the method of delivery or payment has failed；(3) a law now prevents performance； or(4)death or illness prevents performance. " Cf. Bryan A. Gamer, Black's Law Dictionary, 7th edition, at p. 759, West Group (1999).

[2]“Relief from obligation under a contract may be granted when performance has been rendered excessively difficult, expensive, or harmful by an unforeseen contingency. ”Cf. Linda Picard Wood, J. D. Merriam Webster's Dictionary of Law f at p. 238, Merriam- Webster, Incorporated (1996).

[3]“This doctrine provides, generally, that where existence of a specific thing is, either by terms of contract or in contemplation of parties, necessary for performance of a promise in the contract, duty to perform promise is discharged if thing is no longer in existence at time for performance.”Cf. The Publisher's Editorial Staff, Black's Law Dictionary, abridged 6th edition，at p. 462，West Publishing Co. (1991).